It has long been recognized that superior spinnerets for manufacturing manmade fibers should be formed from ceramics, due to these materials' high strength, hardness, wear resistance, temperature resistant, and corrosion resistance. However, these materials are also very difficult and expensive to form and machine, and usually only simple shapes such as circles or cylinders can be formed economically. In addition, spinnerets used in most standard production processes are large, being several inches or more in diameter and up to an inch or more thick, and have formed in them multiple orifices which narrow at the bottom to very small (typical diameters .ltoreq.0.030"), precise holes known as capillaries, and these also are hard to achieve in the structural ceramic materials required for this application. See U.S. Pat. No. 3,825,456 issued to Weber et al, incorporated by reference, for an early attempt at producing these items; however, the method detailed could only be used for ceramic structures formed by recrystallizing an amorphous glass containing mixtures of the oxides of Ti, Al, Li, Mg, Si, etc.--as detailed in U.S. Pat. Nos. 2,684,911, 2,971,853, and 2,920,971, each of which is incorporated herein by reference,--and such structures do not have the strengths required for this application. In addition, it required the spinneret to be assembled from many different pieces, creating the possibility of weaknesses where these are joined.
In addition, many fibers currently being produced are not round, and require very precise, complex shaped capillaries in the spinneret. In the metal spinnerets currently being used, these capillaries are machined by electrical discharge (EDM) and broached, although some non-circular capillaries are also punched. These spinnerets are the most expensive to produce and suffer the most wear and damage in service; therefore, these applications would benefit the most from ceramic spinnerets. However, the most desirable ceramic materials for this application--essentially pure, fine grained technical ceramics such as aluminum oxide, zirconium oxide, silicon nitride, silicon carbide, and engineered composites of these materials--which possess the required strengths and wear resistance, cannot be produced by EDM, or broached, or punched, and only round orifices can be produced by conventional grinding techniques. Thus, while some spinnerets from these ceramic materials have been manufactured, they possessed only round capillaries. U.S. Pat. No. 3,620,703, incorporated herein by reference, details a method of producing non-circular orifices in a ceramic spinneret; however, this requires many individual pieces to be machined and assembled, creating possible weaknesses where the pieces are joined, and also only applies to opacifiable glass which can only produce glass and glass/ceramic structures that do not have the strength required for this application.
Therefore, there exists a need for production, multi-holed spinnerets with non-circular capillaries in essentially pure, fine grain ceramic materials of sufficient strength for this application--aluminum oxide, zirconium oxide, silicon nitride, silicon carbide and composites of these materials. In addition, there is a need for capillaries in these ceramic materials that will fabricate void free shaped fibers and for capillaries in spinnerets of these materials that can fabricate shaped or round fibers containing one or more voids in the fiber, such voids being produced by capillaries consisting of one or more shaped segments, such that the polymer on extrusion knits together during the spinning operation to form a hollow or void containing fiber.